Automobile exhaust gases are conventionally purified with a catalyst supported on a ceramic body able to withstand high temperatures. The preferred catalyst support structure is a honeycomb configuration which includes a multiplicity of unobstructed parallel channels sized to permit gas flow and bounded by thin ceramic walls. The channels may have any configuration and dimensions provided gases can freely pass through them without being plugged by entrained particulate material. Examples of such preferred structures include the thin-walled ceramic honeycomb structures described in U.S. Pat. No. 3,790,654 to Bagley and in U.S. Pat. No. 3,112,184 to Hollenbach.
Ceramic honeycomb catalyst supports are exposed to high temperatures resulting from contact with hot exhaust gases and from the catalytic oxidation of uncombusted hydrocarbons and carbon monoxide contained in the exhaust gas. In addition, catalyst supports must withstand rapid temperature increases and decreases when the automobile engine is started and stopped. Such operating conditions require the ceramic honeycomb catalyst support to have a high thermal shock resistance, a property generally inversely proportional to the coefficient of thermal expansion.
Generally similar ceramic structures are used as diesel engine particulate filters. In such applications, ceramic honeycomb filters are fitted to diesel engine exhaust systems for removal of particulates from the high temperature diesel engine exhaust gases. Examples of diesel engine particulate filters are disclosed in U.S. Pat. Nos. 4,329,162 to Pitcher, Jr. and 4,415,344 to Frost et al. Again, the ceramic materials utilized in such applications must have a high thermal shock resistance and a low coefficient of thermal expansion.
Cordierite (2MgO.2Al.sub.2 O.sub.2.5SiO.sub.2) is known to display a very low thermal expansion over a wide range of temperatures. In substantial amounts, cordierite gives a ceramic body excellent thermal shock resistance when subjected to rapid and severe changes in temperatures. This property has caused cordierite to find widespread use as a catalyst support for automotive catalytic converters and as diesel engine particulate filters. Despite the low average coefficient of thermal expansion of cordierite crystals, the need further to reduce this value in ceramic articles remains a desired objective.
U.S. Pat. No. 3,885,977 to Lachman et al. ("Lachman") forms an extruded, honeycomb carrier having thin walls extending between its ends, and comprising a mixture of clay, talc, and alumina which react during firing to from cordierite. Because of the orientation imparted to the clay and talc platelets during extrusion, the cordierite grains that develop during firing have a preferred orientation, with the cordierite crystallographic c-axes tending to lie in the plane of the walls and the a-axes tending to lie perpendicular to the plane of the walls. The thermal expansion of cordierite is known to be low, even negative, in the direction of the crystallographic c-axis and relatively high in the direction of the a-axis. As a result, a low coefficient of thermal expansion in the direction parallel to the walls and a higher coefficient of thermal expansion transverse the walls is achieved. The low coefficient of thermal expansion aspect of the present invention is able to impart thermal shock resistance to the body as a whole. By contrast, the effect of the transversely-extending high coefficient of thermal expansion regions is minimal, because any expansion in such directions is accommodated by internal spaces in the honeycomb.
Others have attempted to produce cordierite bodies with an improved coefficient of thermal expansion. For example, in U.S. Pat. No. 3,958,058 to Elmer, cordierite with an ultra-low thermal expansion coefficient and a high thermal shock resistance is obtained by treating it with a strong mineral acid. The acid is said to remove Al.sub.2 O.sub.3 and MgO, while the low expansion values are attributable to microcracks in the leached material. It has been elsewhere recognized that the internal stresses in highly anisotropic crystalline cordierite ceramics leads to such microcracking. Despite its ability to reduce a cordierite article's coefficient of thermal expansion, this acid treatment has not received wide acceptance, because it requires specialized handling systems for the acid and chemically modifies the cordierite product by removal of Al.sub.2 O.sub.3 and MgO.
U.S. Pat. No. 3,979,216 to Fritsch, Jr. et al. relates to the production of synthetic cordierite ceramics having thermal expansions below 1100 ppm in the temperature range of 25.degree.-800.degree. C. and 15 to 150 micrometer microcracks. This product is prepared by mixing talc, clay, and alumina, consolidating this mixture as a green body, heating at 150.degree. C. per hour to a temperature of 1350.degree. to 1425.degree. C., holding at this elevated temperature for 0.5 to 10 hours, and cooling to below 1000.degree. C. at a rate of -150.degree. C. per hour. Thermal expansion is said to be dependent upon maintaining a low glass content in the body.
U.S. Pat. No. 4,869,944 to Harada et al. relates to a cordierite honeycomb structural body formed by including high purity, non-crystalline silica in a mixture of talc, kaolin, and alumina. As a result, crystals with their c-axes preferentially oriented in the plane of the walls are formed. Structural microcracking occurs to the same degree regardless of whether or not high purity, non-crystalline silica is incorporated in the mixture; however, the use of silica causes more microcracks to form along the crystallographic c-axis direction of the crystals in the domain structure. With such microcrack orientation, the article is able to absorb positive thermal expansion better and give the body a low coefficient of thermal expansion.
Although there have been many efforts to reduce the coefficient of thermal expansion of cordierite-containing articles in a variety of ways, there has been little work done to optimize the precursors used to form a cordierite-containing article. There is a need for improved raw material batch compositions that will improve the coefficient of thermal expansion of the cordierite body while maintaining the other favorable properties cordierite typically exhibits.